Since its introduction in the 1930s, transmission electron microscopy (TEM) has found broad application in various different branches of science and economics. Due to the significantly improved resolution capability compared to light microscopy, the microstructure and nanostructure of various different preparations can be examined in great detail.
Atom-probe tomography (LEAP) is also used for chemical characterizations on the smallest length scales. This method for microstructure diagnostics enables a two-dimensional image and, moreover, supplies three-dimensional maps of the local composition with atomic resolution.
With increasing capability of the methods for microstructure diagnostics, the question regarding efficient and low-damage methods for preparing samples for these methods increasingly arises.
The problem of preparing cross-sectional samples often arises in the field of semiconductor technology and thin-layer technology, but also in other fields of technology. In contrast to a volume sample, a cross-sectional sample is a sample intended to serve for undertaking microstructure examinations in the region of interfaces between different materials adjoining one another in the region of an interface, e.g. in components with a layer structure.
Substantially two routes are followed to generate electron-transparent cross-sectional preparations, namely (i) the use of focused ion beam (FIB) systems to generate samples directly from the surface of a substrate by focusing ion beam technology and (ii) production of samples on the basis of sandwich bondings subsequently finished mechanically and then finally thinned by an Ar wide beam.
Within the last decade, preparation of cross-sectional samples for transmission electron microscopy in the form of FIB lamellas has found wide use in virtually all fields of microstructure analysis due to its great target accuracy. In the field of metrology and structure elucidation in highly integrated semiconductor components, it is currently considered to be de facto the only practically applicable method due to the achievable target accuracy (a few 10 nm).
However, fundamental physical restrictions lead to the high processing precision being accompanied by a low ablation rate. It is for this reason that only very small sample bodies with dimensions in the region of a few tens of micrometers can be prepared by FIB technology. Therefore, FIB generated sample bodies are mounted on carrier structures compatible with standardized sample holders of TEM installations, for the subsequent TEM analysis. For the purposes of the transfer, use is made of ex situ and in situ lift out techniques using micro- and nano-manipulators.
It is disadvantageous in that procedure that (i) the FIB installation is re-functioned from a precise processing tool to an expensive handling tool under vacuum conditions, as result of which the instrument capacity for processing is reduced, (ii) high additional costs are required for manipulator systems with sufficient precision in addition to the high acquisition costs of the actual FIB installation, (iii) there is a certain amount of risk that the susceptibility to errors of the overall system is increased by the complexity of the micro- and nano-manipulators, and (iv) the complexity of the overall workflow requires very well educated and experienced operators.
Methods for sample preparation operating with a combination of laser beam processing and ion beam processing have already been proposed as well. A sample body with a predeterminable form is prepared from a substrate by way of material-ablating laser beam processing and, subsequently, a target portion of the sample body is further processed by way of laser beam processing and/or ion beam processing to expose a target volume suitable for a microstructure examination. Those methods do not have the weakness of low ablation rates from FIB micro-processing arising as a matter of principle.
DE 10 2011 111 190 A1 describes a method of preparing a sample for microstructure diagnostics in which a flat disk is irradiated along two opposite surfaces thereof by a high-energy beam such that a recess extending approximately parallel to a central disk plane is introduced by radiation-induced material ablation into the two surfaces, with the two recesses extending on both sides of the central disk plane being introduced such that the longitudinal axes thereof, when seen in a projection of the longitudinal axes on this central disk plane, intersect at a predetermined finite angle and that, as seen perpendicular to the central disk plane, a material portion with a predefined minimum thickness, which is preferably already transparent to an electron beam, remains in the region of intersection of the two recesses and between the recesses as a sample. After the laser processing, the region of a low thickness can be thinned further by ion beam etching.
EP 2 787 338 A1 describes a method of preparing a sample for microstructure diagnostics in which a base structure consisting of the substrate material is isolated from a flat substrate radiating-in a laser beam in a manner perpendicular and/or oblique to the substrate surface, the base structure comprising a carrier structure and, integrally therewith, a structure carried by the carrier structure. By way of example, the carrier structure can have a C-shaped design, while the carried structure can be a thin bar-shaped target portion between the ends of the C-shaped carrier structure. The thickness of the target portion—as measured perpendicular to the substrate surface—corresponds to the substrate thickness. The side faces of the target portion extend parallel to the substrate surface. The target volume of interest lies in the target portion and it is isolated by further laser beam processing and subsequent ion beam processing after removing the base structure from the residual substrate and subsequently clamping the removed base structure into a clamp mounting. During laser beam processing, the laser beam is radiated-in in parallel or at an acute angle with respect to the side faces of the plate-shaped target portion such that e.g. electron-transparent regions arise, which can be transilluminated perpendicular to the former substrate surface.
The two methods are very well suited for quick and reliable preparation of volume materials. It is likewise possible to realize cross-sectional preparations by appropriate finishing of the initial material (e.g. sandwich bonding and subsequent mechanical comminution by sawing or grinding). However, there is increased outlay in terms of time. Moreover, experience of the user is required for good target accuracy.
It could therefore be helpful to provide a minimally invasive, reproducibly reliable, quick method with few artifacts for the targeted preparation of samples for microstructure diagnostics, suited equally to cross-sectional samples and volume samples and to prepare samples of the highest quality for cross-sectional transmission electron microscopy (X-TEM) within a relatively short period of time.